In recent years, from the social requirement of lower fuel consumption and less exhaust emission, an electric automobile and a hybrid automobile, each of which is mounted with an AC motor as a power source of a vehicle, is drawing attention. For example, in some of the hybrid automobiles, a DC power source made of a secondary battery or the like and an AC motor are connected to each other via an electric power conversion device constructed of an inverter and the like, and a DC voltage of the DC power source is converted into an AC voltage by the inverter to thereby drive the AC motor.
In the control device of the AC motor mounted in this kind of hybrid automobile and electric automobile, there has been known a technique in which a current sensor for sensing a phase current is provided in one phase to thereby reduce the number of current sensors, whereby a construction near three phase output terminals of an inverter can be reduced in size and a control system of the AC motor can be reduced in cost (for example, see patent document 1).
In the patent document 1, one phase control is performed in the following manner: a current sensor value of one phase is used for the one phase; and for other phases, three phase AC current command values, which can be acquired by inversely dq transforming a d axis current command value and a q axis current command value on the basis of an electric angle, are used as current estimated values of the other phases. Three phase AC current command values acquired by inversely dq transforming the d axis current command value and the q axis current command value does not become information correctly reflecting the actual current of the AC motor, and hence the control of the AC motor is likely to become unstable. In particular, when the number of revolutions of the AC motor is small, a current change in a current sensed value and a rotation angle movement per a sampling interval become small and hence actual information is scarcer, which hence is likely to make the control of the AC motor more unstable.    [Patent Document 1] JP-A No. 2008-86139 (corresponding to US 2008/0079385-A1)